Furnaces having combustion chambers usually transfer their heat of combustion to supply air by means of a clamshell type heat exchanger. The heated supply air is then conveyed to any rooms that need heat. However, a substantial amount of heat is often wasted because the products of combustion comprising flue gas are still relatively hot as they exhaust to atmosphere from the clamshell heat exchanger. Therefore, attempts have been made to recover this waste heat to improve the overall efficiency of the furnace.
U.S. Pat. No. 4,020,822 shows one method of recovering this waste heat by placing the flue gas in heat exchange relation with the incoming supply air. With this method, the flue gas preheats the supply air before it passes across the clamshell heat exchangers and into the rooms needing heat. However, redirecting the supply air flow across a flue gas heat exchanger can often involve a major redesign of a conventional furnace.
Another heat recovery method, which has often been overlooked, involves using the flue gas to preheat the intake combustion air. U.S. Pat. Nos. 3,307,471 and 3,429,307 both show a flue gas passage adjacent to a combustion air passage. Neither patent, however, mentions any heat exchange relation between the two, nor makes any attempt to take full advantage of their proximity since they are positioned adjacent to each other for purposes other than for heat transfer. The '471 patent even includes insulation (Item 65 of FIG. 5) that insulates a potential heat exchange surface that could otherwise be used to preheat combustion air.
Other methods, such as those disclosed in U.S. Pat. Nos. 1,785,334; 1,789,401; and 4,364,514 include separately mounted, elaborate heat exchangers rather than simply making use of existing conventional flue ducts which are internally mounted within the furnace enclosure.
In view of the shortcomings of earlier methods, it is an object of the subject invention to take advantage of the temperature differential between the hot flue gas and relatively cool intake combustion air by placing them in heat exchange relation with each other.
Another object is to increase the heating efficiency of a furnace by recovering heat from hot combustion flue gas that would otherwise be wasted.
Another object is to preheat the intake combustion air entering the combustion chamber of the furnace.
Yet another object is to provide a flue gas/combustion air heat exchanger that is disposed inside the furnace enclosure.
A further object is to use a conventional flue gas duct to not only convey flue gas in a direct, non-tortuous path for minimal flow resistance, but also to function as a heat exchanger.
A still further object is to position a combustion air blower such that is discharges across a hot flue gas duct in a direct, non-tortuous path for minimal flow resistance.